Capacitors with electrodes chemically modified by, for example, the immobilization of conducting polymers on the surface of inert electrodes are one of the promising types of electrochemical capacitors.
Conducting polymers are divided into two types [B. E. Conway, Electrochemical Supercapacitors//Kluwer Acad. Plen. Publ., NY, 1999, 698 p].
The so-called “organic metals” or conducting polymers—the polymers with the type of conductivity which is similar to the mechanism of conductivity of metals;
Redox polymers—the compounds in which the transfer of electrons occurs mainly due to the oxidation-reduction chemical reactions between the adjacent fragments of a polymer chain.
Examples of the “organic metals” are polyacetylene, polypyrrole, polytiophen, polyaniline. In a partially oxidized form, these polymers exhibit even higher conductivity and can be regarded as a salt consisting of positively charged “ions” of the polymer and counter-ions that are uniformly distributed throughout its structure and that support the overall electrical neutrality of the system. In solid state physics a cation-radical partially delocalized by polymer fragment is called a polaron. A polaron stabilizes, thus, polarizing the ambient medium. The polaron theory of conductivity is accepted as the main model of charge transfer in conducting polymers [Charge Transfer in Polymeric Systems//Faraday Discussions of the Chemical Society. 1989. V.88].
“Organic metals” may be produced by electrochemical oxidation of the appropriate monomers on the surface of the inert electrode. These polymers may be converted from the conducting (oxidized) state to the non-conducting (reduced) state via the variation of the electrode potential. The transfer of the polymer from the oxidized state to the neutral reduced state is accompanied by the departure of the charge-compensating counter-ions from the polymer to the electrolyte solution in which the process takes place, and vice versa.
The redox polymers comprise both purely organic systems and polymer metal complexes or metal organic compounds [H. G. Cassidy and K. A. Kun. Oxidation Reduction Polymer//Redox Polymers. Wiley—Interscience, New York, 1965]. The metal-containing polymers exhibit the highest conductivity.
As a rule, polymer metal complex compounds are obtained via electrochemical polymerization of the source monomer complex compounds, which have an octahedral configuration, the polymerization occurring on inert electrodes. The spatial configuration of the monomers plays a principle role in the formation of the polymer structures suitable for application in the capacitors. As an example of the redox polymers obtained from the octahedral source complex compounds, it is possible to consider polypyridine complexes of composition poly-[Me(v-bpy)x(L)y], where.
Me=Co, Fe, Ru, Os;
v-bpy=4-vinyl-4′-methyl-2,2′-bipyridine;
L=v-bpy (4-vinyl-4′-methyl-2,2′-bipyridine), phenanthroline-5,6-dione, 4-methylphenanthroline, 5-aminophenanthroline, 5-chlorphenanthroline;
at that (x+y=3) [Hurrel H. C., Abruna H. D. Redox Conduction in Electropolymerized Films of Transition Metal Complexes of Os, Ru, Fe, and Co/Inorganic Chemistry. 1990. V.29 P.736–741].
Ions of metal, which may be in different charge states, serve as redox centers, i.e. atoms, which participate in the oxidation-reduction reactions in a polymer. Thus, complexes of metals having only one possible charge state (for example, zinc, cadmium) don't create redox polymers. The presence of the complexes of branched system of conjugated p-bonds bonds serve as conducting “bridges” between redox centers in the ligand environment is a necessary condition of conductivity of redox polymers. When a redox polymer is completely oxidized or completely reduced, i.e. when all its redox centers are in one charge state, the transfer of the charge along the polymer chain is impossible and the conductivity of the redox polymer is close to zero. When the redox centers have different charge states, the exchange of electrons between them becomes possible in the same manner as it happens in the solutions during the oxidation-reduction reactions. Therefore, the electric conductivity of redox polymers is proportional to the constant of the electron self-exchange rate between redox centers (kco) and to the concentrations of oxidized and reduced centers ([Ox] and [Red]) in a polymer, i.e. the conductivity of a redox polymer is ˜kco[Ox) [Red].
The conductivity of redox polymers is maximumal at equal concentrations of oxidized and reduced redox centers, which corresponds to the conditions when a redox system has a standard oxidation-reduction potential equal to E° ([Ox]/[Red]).
The existence of the redox centers in different charge states was the reason for naming the redox polymers based on coordination compounds “Mixed valence complexes” or “partly oxidized complexes”. The transfer of redox polymer molecules from the oxidized state to the reduced state is accompanied (like it was described above in reference to conducting polymers) by the transition of charge-compensating counter-ions from the polymer into the electrolyte solution where the process takes place, and vice versa.
Compared to the electrodes modified by “organic metals” (conducting polymers), redox polymers and electrodes with redox polymers on their surface can potentially offer higher specific energy capacity due to a higher contribution of the Faraday component of the capacitance (related to the possibility of multi-electron oxidation/reduction of metal centers) to the overall capacitance of the polymer.
One of the disadvantages of electrochemical capacitors with chemically modified electrodes based on the redox polymer of a poly-[Me(R-Salen)] type is the deterioration of the electrochemical characteristics of their electrodes and the corresponding impairment of electric characteristics of the capacitors. This occurs due to a partial dissolution of a redox polymer in the electrolyte during operation of such a capacitor.